Why are my handlebars vibrating?

Subject basically says it all.

About a week ago, my handlebars suddenly started vibrating when riding
hands-free. This happened overnight. Rode Mon, no problem. Tue,
vibration. Speed seems to be irrelevant. I *can* ride hands-off without
vibration if I keep the ride absolutely smooth but as soon as I pedal or
hit the slightest bump, it starts.

I'm strongly considering replacing the fork for other reasons, would that
be likely to fix this too?

Thanks,
Mark

--
Remove both wrongs to make the email address right.

Mark Mitchell wrote in news:k9z8d.106103
:
About a week ago, my handlebars suddenly started vibrating when riding
hands-free. This happened overnight. Rode Mon, no problem. Tue,
vibration. Speed seems to be irrelevant.

Is your headset loose?

On 2004-10-05, Ken wrote:
Mark Mitchell wrote in news:k9z8d.106103
:
About a week ago, my handlebars suddenly started vibrating when riding
hands-free. This happened overnight. Rode Mon, no problem. Tue,
vibration. Speed seems to be irrelevant.

Is your headset loose?


I don't think so. I'd thought of that, tested by holding the front wheel
between my legs, locking the front brake and wiggling forward and back,
looking for play.

Nothing.

If that's not the way to be sure, then I don't know.

Mark

--
Remove both wrongs to make the email address right.

Mark Mitchell writes:

About a week ago, my handlebars suddenly started vibrating when
riding hands-free. This happened overnight. Rode Mon, no problem.
Tue, vibration. Speed seems to be irrelevant. I *can* ride
hands-off without vibration if I keep the ride absolutely smooth but
as soon as I pedal or hit the slightest bump, it starts.

If I understand your description, this is shimmy, an oscillation of
the front wheel at about shivering frequency. How long have you
ridden this bicycle in a similar fashion with no shimmy?

I'm strongly considering replacing the fork for other reasons, would
that be likely to fix this too?

How tall are you? The distance between saddle, pedals, and front
wheel ground contact have an effect on incidence of shimmy.

Subject: 8h.5 Shimmy or Speed Wobble
From: Jobst Brandt
Date: Mon, 25 June 2004 14:13:14 PDT

Shimmy, a spontaneous steering oscillation of the front wheel, usually
occurs at a predictable speed when riding no-hands. The likelihood of
shimmy is greatest when the only rider-to-bicycle contact is at the
saddle and pedals. This position gives the least damping by hands,
arms, and legs. When shimmy occurs on descents, with hands on the
bars, it is highly disconcerting because the most common rider
response, of gripping the bars firmly, only increases it.

Shimmy is not related to frame alignment or loose bearings, as is
often claimed. Shimmy results from dynamics of front wheel rotation,
mass of the handlebars, elasticity of the frame, and where the rider
contacts the bicycle. Both perfectly aligned bicycles and ones with
wheels out of plane to one another shimmy nearly equally well. It is
as likely with properly adjusted bearings as loose ones. The idea
that shimmy is related to bearing adjustment or alignment has been
established by repetition.

Bicycle shimmy is the lateral oscillation of the head tube about the
road contact point of the front wheel and depends largely on frame
geometry and the elasticity of the top and down tubes. It is driven
by gyroscopic forces of the front wheel, making it largely speed
dependent. It cannot be fixed by adjustments because it is inherent
to the geometry and elasticity of the bicycle frame. The longer the
frame and the higher the saddle, the greater the tendency to shimmy,
other things being equal. Weight distribution also has no effect on
shimmy although where that weight contacts the frame does. Bicycle
shimmy is unchanged when riding no-hands, whether leaning forward or

Shimmy requires a spring and a mass about which to oscillate and these
are furnished by the frame and seated rider. Unloading the saddle
(without standing up) will stop shimmy. Pedaling or rough road will
also reduce the tendency to shimmy. In contrast, coasting no-hands
downhill on a smooth road at more than 20mph with the cranks vertical
seems to be the most shimmy prone condition.

When coasting no-hands, laying one leg against the top tube is the
most common way to inhibit shimmy and also one of the most common ways
to coast no-hands. Compliant tread of knobby tires usually have
sufficient squirming damping to suppress shimmy. Weight of the
handlebar and its extension from of the steering axis also affects
shimmy.

Shimmy is caused by the gyroscopic force of the front wheel whose tilt
is roughly at right angles to the steering axis, making the wheel
steer to the left when it leans to the left. This steering action
twists the toptube and downtube, storing energy that both limits
travel and causes a return swing. Trail (caster) of the fork acts on
the wheel to limit these excursions and return them toward center.

Shimmy that concerns riders the most occurs with hands firmly on the
bars and it is rider generated by muscular effect whose natural
response is the same as the shimmy frequency, about that of Human
shivering. Descending in cold weather can be difficult for this
reason. The rider's "death grip" only enhances the incidence of
shimmy in this situation. Loosely holding the bars between thumb and
forefinger is a way of avoiding shimmy when cold.

------------------------------
Jobst Brandt

On 2004-10-05, wrote:
Mark Mitchell writes:

About a week ago, my handlebars suddenly started vibrating when
riding hands-free. This happened overnight. Rode Mon, no problem.
Tue, vibration. Speed seems to be irrelevant. I *can* ride
hands-off without vibration if I keep the ride absolutely smooth but
as soon as I pedal or hit the slightest bump, it starts.

If I understand your description, this is shimmy, an oscillation of
the front wheel at about shivering frequency. How long have you
ridden this bicycle in a similar fashion with no shimmy?

I've owned the bike for approx 5yrs, but if I had to guess, I'd say in that
time I've put maybe 1500 miles on it.

I'm strongly considering replacing the fork for other reasons, would
that be likely to fix this too?

How tall are you? The distance between saddle, pedals, and front
wheel ground contact have an effect on incidence of shimmy.

I'm 5'6" on a 42cm Trek 6000.

The bike is basically stock. I've added a rear rack, cheap bike computer,
front and rear lights and rear fender. The onset of the shimmy was not
associated with any parts changes on the bike. The load on the rack
doesn't seem to be associated in any way with the shimmy.

snip

On reading your article, the only thing that jumps out at me as relevant is
cold.

Is it possible that colder weather can change the elasticity of the frame
in such a way as to allow a shimmy below 40-50 and not above.

I noticed riding home today in 60-65 degree weather that it had a barely
perceptible tendancy to wiggle.

Hope that's not the case, I'd planned on riding this bike through as much
of a MN winter as possible.

Mark

This exact problem happend to me. I found jobst brant explanation to
be precise. My solution was new bike frame. My problem seemed to get
worse after a couple of weeks. Apparently this happens more frequently
to larger riders. Good luck.

On Tue, 05 Oct 2004 21:37:30 GMT, Mark Mitchell
wrote:

On 2004-10-05, wrote:
Mark Mitchell writes:

About a week ago, my handlebars suddenly started vibrating when
riding hands-free. This happened overnight. Rode Mon, no problem.
Tue, vibration. Speed seems to be irrelevant. I *can* ride
hands-off without vibration if I keep the ride absolutely smooth but
as soon as I pedal or hit the slightest bump, it starts.

If I understand your description, this is shimmy, an oscillation of
the front wheel at about shivering frequency. How long have you
ridden this bicycle in a similar fashion with no shimmy?

I've owned the bike for approx 5yrs, but if I had to guess, I'd say in that
time I've put maybe 1500 miles on it.

I'm strongly considering replacing the fork for other reasons, would
that be likely to fix this too?

How tall are you? The distance between saddle, pedals, and front
wheel ground contact have an effect on incidence of shimmy.

I'm 5'6" on a 42cm Trek 6000.

The bike is basically stock. I've added a rear rack, cheap bike computer,
front and rear lights and rear fender. The onset of the shimmy was not
associated with any parts changes on the bike. The load on the rack
doesn't seem to be associated in any way with the shimmy.

snip

On reading your article, the only thing that jumps out at me as relevant is
cold.

Is it possible that colder weather can change the elasticity of the frame
in such a way as to allow a shimmy below 40-50 and not above.

I noticed riding home today in 60-65 degree weather that it had a barely
perceptible tendancy to wiggle.

Hope that's not the case, I'd planned on riding this bike through as much
of a MN winter as possible.

Mark

If you do plan to ride through the winter, I'd suggest a change to a
different tire, as the differing mass can change or eliminate the
shimmy. Perhaps some cross tires to take advantage of the knobby
squirm which Jobst points out supresses shimmy.

I have a commuter/mountain bike with 3 sets of wheels/tires- shimmies
with one set, not at all woth the others.

Mark Mitchell writes:

I'm 5'6" on a 42cm Trek 6000.

That is not in the range where shimmy becomes more pronounced. With
my height, over 6' 5" all the bicycles I have owned have shimmied
reliably if I let them, although they had also coasted (hands on stem)
stably at up to 60mph. That was my reason for asking.

The bike is basically stock. I've added a rear rack, cheap bike
computer, front and rear lights and rear fender. The onset of the
shimmy was not associated with any parts changes on the bike. The
load on the rack doesn't seem to be associated in any way with the
shimmy.

I just wondered whether there was any event or change that might
influence the bicycle's stability.

On reading your article, the only thing that jumps out at me as
relevant is cold.

Is it possible that colder weather can change the elasticity of the
frame in such a way as to allow a shimmy below 40-50 and not above.

Metals do not change elasticity in that range and what they do is so
small as to be immaterial to frame elasticity. For metals -30F is
cold. Avoid riding in such weather for your own well being. Frost
bite is a permanent injury.

I noticed riding home today in 60-65 degree weather that it had a
barely perceptible tendency to wiggle. Hope that's not the case,
I'd planned on riding this bike through as much of a MN winter as
possible.

Not to worry, it is the riders response that can enhance shimmy in
cold weather, not that of the bicycle. The way you describe it, I
can't imagine what causes your bicycle to shimmy now and not
previously.

Jobst Brandt